The present invention relates to cast steel and more particularly, to medium chromium (Cr) low nickel (Ni) stainless cast steel especially having stable corrosion resistance against strong acids and a method of producing said cast steel.
Recently, with the remarkable development in the fields of chemical industry, paper-manufacturing industry, etc., requirements for materials having stable corrosion resistance have been increasing more and more. Although bronze has conventionally been employed extensively as a reliable material having sufficient corrosion resistance against strong acids, it has problems related to designing with respect to the demand for larger size of various facilities, due to its low allowable stress, elastic modulus and yield strength for structural material requiring high strengths. Accordingly, martensite stainless steel of 13 Cr group is generally employed for such purposes, while stainless steels of 18-8 and 18-8-Mo groups are generally used in circumstances subjected to influence of strong acids. Meanwhile, in the field of stainless steel, high Cr low Ni two phase stainless steel having higher strength and corrosion resistance than the conventional stainless steels have been developed, and recently found it way in the application to tubes for sea water heat exchangers, rolls for paper manufacturing, etc.
The two phase stainless steel as described above, however, has not yet been put into wide actual use, with various characteristics thereof still being left to be found. Accordingly, at the present stage, stainless steel of 18-8 group or 18-8-Mo group mentioned earlier is mainly used, but since the stainless steel of the above kind has allowable stress lower than the 13 Cr steel, it has not been brought into actual use with full confidence under the present circumstances.
Accordingly, an essential object of the present invention is to provide a stainless cast steel of high corrosion resistance and high strength medium Cr low Ni group which is extremely superior in yield strength to conventional stainless steels of 18-8 group or 18-8-Mo group and can sufficiently be applicable to various circumstances influenced by strong acids, with substantial elimination of disadvantageous inherent in stainless cast steels of the kind.
Another important object of the present invention is to provide a stainless cast steel of the above described type which is stable and reliable in structure and preformance, and can be readily manufactured through simple processings at low cost.
A further object of the present invention is to provide a method of producing a stainless cast steel of the above described type.
In accomplishing these and other objects, according to one preferred embodiment of the present invention, the high corrosion resistant and high strength medium Cr and low Ni stainless cast steel includes, in weight percentage, C (carbon): 0.1% and below, Si (silicon): 1.5% and below, Mn (manganese): 2.0% and below, P (phosphorus): 0.04% and below, S (sulphur): 0.04% and below, Cr (chromium): 17.0% to 20.0%, Ni (nickel): 3.0% to 7.0%, Mo (molybdenum): 1.5% to 2.5%, Cu (copper): 5.0% to 7.0%, N (nitrogen): 0.1% and below and remaining portion substantially of Fe to form material for the stainless cast steel when said material is further subjected to heat treatment under predetermined conditions as disclosed, stainless steels superior in yield strength and having stable corrosion resistance against strong acids are presented, with substantial elimination of disadvantages inherent in conventional stainless steels.